1. Field of the Invention
The present invention relates to a liquid crystal, an alignment layer and an alignment process, which are applied to a liquid-crystal display device. More particularly, the present invention relates to a process and an apparatus for measuring pretilt angle of liquid crystals, which are applied to a TFT liquid-crystal display device and a STN liquid-crystal display device.
2. Description of the Prior Art
Two kinds of methods for measuring a pretilt angle, which means an angle of liquid-crystal director to a substrate face, are well used. One of them is known as a magneto-capacitive null method and the other is known as a crystal rotation method. These measuring methods are described in detail in Journal of Physics, Vol. 48, No. 5, p. 1783-1792 (1977).
The magneto-capacitive null method is described as follows: A liquid-crystal cell is prepared. Then a capacity is measured while the liquid-crystal cell is rotated. When the same capacity as the one measured in no magnetic field is given, the rotation angle is referred to as a pretilt angle.
The crystal rotation method is described as follows: A liquid-crystal cell is prepared. Then polarized light is emitted into the liquid-crystal cell at angle of 45 degrees to a rubbing direction. The cell is rotated and light-transmittance is measured. A fringe of light-transmittance to a rotation angle becomes symmetric at a certain degree. A pretilt angle is calculated from the wavelength of the light, the symmetric angle and refractive index of liquid crystals.
The above methods for measuring a pretilt angle have the following defects. In the magneto-capacitive null method, it is hard for liquid crystals to align in a magnetic field unless a thickness of liquid-crystal cell is 10 .mu.m or more, as a result of which it becomes difficult to measure a pretilt angle. In the crystal rotation method, it is difficult to determine a symmetric angle accurately unless a thickness of liquid-crystal cell is 10 .mu.m or more, as a result of which a pretilt angle can not be measured.